Method for producing stainless steel

ABSTRACT

IN THE PRODUCTION OF STAINLESS STEELS INVOLVING ADDITIONS OF TITANIUM, COLUMBIUM OR TANTALUM, THE SLAG IS REMOVED FROM THE MOLTEN STEEL BEFORE TAPPING INTO THE LADLE. THE MELT IS SUBJECTED TO VACUUM AND AGITATED BY THE INTRODUCTION OF AN INERT GAS THROGH A POROUS PLUG NEAR THE BOTTOM OF THE LADLE. DURING SUCH TREATMENT, THE TITANIUM, COLUMBIUM OR TANTALUM IS ADDED UNDER VACUUM AND MIXED WITH THE MELT BY THE INERT GAS AGITATION. FOLLOWING TREATMENT, THE MELT IS PRESSURE POURED INTO A MOLD BY DISPOSING A POURING TUBE BETWEEN A LOWER PORTION OF THE MELT AND A MOLD, INCLOSING THE POURING TUBE AND LADLE IN A TANK, AND APPLYING SUPERATMOSPHERIC PRESSURE WITHIN THE TANK.

pl'l 13, 1971 G, R, OHMAN ETAL 3,574,596

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. METHOD FOR PRODUCING STAINLES-S STEEL Filed Sept. 15, 1967 3 Sheets-Sheet 3 FIGURE 6 Gannon n. LoHMAN mcmmn x. MATuscnxovlTz United States Patent O 3,574,596 METHOD FOR PRODUCING STAINLESS STEEL Gordon R. Lohman, Glen Ellyn, and Richard Kurt Matuschkovitz, Chicago, Ill., assignors to Amsted Industries Incorporated, Chicago, Ill.

Filed Sept. 15, 1967, Ser. No. 667,978

Int. Cl. C21c 7/00 U.S. Cl. 75-49 4 Claims ABSTRACT F THE DISCLOSURE In the production of stainless steels involving additions of titanium, columbium or tantalum, the slag is removed from the molten steel before tapping into the ladle. The melt is subjected to vacuum and agitated by the introduction of an inert gas through a porous plug near the bottom of the ladle. During such treatment, the titanium, columbium or tantalum is added under vacuum and mixed with the melt by the inert gas agitation. Following treatment, the melt is pressure poured into a mold by disposing a pouring tube between a lower portion of the melt and a mold, inclosing the pouring tube and ladle in a tank, and applying superatmospheric pressure Within the tank.

This invention relates to methods of producing stainless steel and more particularly to a method for reducing inclusions normally found in certain grades of stainless steel.

In the production of those stainless steels containing additions of titanium, columbium, or tantalum, the formation of carbonitride inclusions with these elements presents a serious deterrent to wide commercial usage. During the casting operation, such inclusions tend to rise and concentrate near the top of the slab or other article being cast and impart undesirable properties to the final product. For example, upon the rolling of a stainless steel slab produced by conventional methods, defects such as slivers and scratches occur due to the carbonitride inclusions. Moreover, since these inclusions tend to concentrate in one portion of the slab, a majority of the defects will be concentrated in a relatively small area, resulting in a highly unsatisfactory product.

Very few methods have been proffered to avoid the problems stated above. One proposed solution involves a reladling technique, wherein the alloying material is added during tap. The melt is then slagged olf and poured back into another ladle to achieve some degree of mixing and separation of inclusions suspended in the melt. This technique is generally inefficient and may result in substantial heat losses, thereby necessitating reheating. Another method involves the removal of the inclusion-enriched portion of the slab, which may result in considerable loss of material.

Accordingly, an object of this invention is to provide a method of reducing the amount of carbonitride agglomerations normally found in stainless steels containing titaniium, columbium, or tantalum.

Another object of this invention is to provide a combination method for treating and casting iron base melts that will be effective to minimize atmospheric contamination subsequent to treatment.

The above and other objects will become apparent to those skilled in the art from the following description and appended claims, and in connection with the accompanying drawings wherein:

FIG. 1 is a cross-sectional schematic illustration of the apparatus used in conjunction with the first portion of the presently described process;

FIG. 2 is a cross-sectional schematic illustration of the apparatus used in conjunction with the remainder of the presently described process; and

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FIGS. 3, 4, 5 and 6 represent results of experiments herein described.

With reference to the drawings, FIG. l illustrates a substantially sealed tank 10 housing a ladle 12 which contains molten steel 14. Preferably, substantially all of the slag has been removed from the molten steel before tapping from the furnace into the ladle. The chemical composition of the molten steel will, of course, be dependent upon its intended application. For example, in the production of type 321 stainless, the steel will contain the following approximate maximum percentages of constituents: 0.08 carbon; 2.00 manganese; 1.00 silicon; 0.045 phosphorus; and 0.030 sulphur. In addition, the steel will contain from about 17 to 19 percent chromium, 9 to 12 percent nickel, and an amount of titanium equal to at least five times the carbon content.

In conventional operations, the above constituents are melted in the furnace and transferred by ladle to a mold. Due to the dissolved nitrogen in the molten bath, however, the titanium, tantalum or columbium may react to form carbonitride agglomerations that are only partially absorbed by the slag. Ideally, these agglomerations should be minimized by removing nitrogen from the melt or by uxing the particles out of the bath, in order to insure a final product of consistent and uniform quality. However, as mentioned before, such a result has not been suiciently attainable by utilizing presently known methods.

The present invention contemplates removal of a substantial portion of the nitrogen dissolved in the molten steel by vacuum degassing, as shown in FIG. 1. A vacuum line 16 is connected to the tank 10 and to a vacuum pump indicated diagrammatically at 18. Also, a porous plug 20 is positioned in an interior wall of the ladle 12 in the lower portion thereof. A sealed hopper 21 containing alloying materials 22 communicates through the cover 2'4 of the tank 10.

After the ladle 12 has been lled with the molten metal 14, cover 24 is placed on tank 10 and a vacuum is drawn through line 16, in order to remove dissolved nitrogen from the molten steel. At the same time, the molten metal is mixed by the introduction of an inert gas, such as argon, from a source 26 through porous plug 20 at a relatively low rate. The molten metal 14 in ladle 12 contains all of the necessary constituents required except for the titanium, columbium or tantalum, which is later introduced through the hopper 20.

The molten steel is preferably subjected to vacuum for about five to fifteen minutes, and the inert gas is introduced at a rate of 35 to 65 c.f.h., depending on the size of the ladle. In any event, it is preferable that the gas llow be maintained at a low enough level such that the metal does not splatter or foam excessively. After about half of the vacuum time has elapsed, the previously Withheld alloying materials mentioned above are then added under vacuum.

After the alloying materials mentioned above have been added, the continued bubbling of inert gas thereafter serves to mix the alloy thoroughly with the melt and enhances equilibrium between the alloy and any nitrogen remaining in the melt. Moreover, the melt agitation causes the carbonitride particles to agglomerate into larger particles that more readily rise to the surface of the melt to 'be absorbed by any slag present at the top of the bath.

After the aforesaid alloying materials have been added and the vacuum treatment completed, the cover 24 is removed and the molten steel is pressure poured, as shown in FIG. 2. In this instance, the substantially sealed tank 10 is connected to a pneumatic uid pressure line 28, adapted to deliver pressurized pneumatic uid such as compressed air to the tank.

The top of the tank 10 is closed by a removable cover 30 seated on a resilient seal ring 32 and secured to the tank by releasable clamps or other suitable means (not shown) to afford a substantially sealed enclosure. Cover 30 has an opening 34 adapted to receive a pouring tube 36 that extends through the cover into a lower portion of the ladle 12. A mold 38 is provided above the cover 30 of the tank 10 and is mounted on the upper extremity of the pouring tube 36. Molten metal 14 may be forced upwardly through the pouring tube 36 into mold 38 by the application of superatmospheric pressure on the molten metal in ladle 12.

The mold 38 shown in FIG. 2 is a slab mold and comprises a plurality of blocks including a top block 40, a bottom block 41, an end block 42 and two side blocks, one of -which is indicated at 43. The blocks are held in engagement by suitable means (not shown) to dene .Time, minutes: seconds Temperature, F.

Start tap Finish tap Vacukilzm applied and start argon bubbling at 46 3 250 c 0.40% Ti added VacuuJn terminated and argon ot Vacuum cover off Pressure cover on Completion of pressure pour. Slab removed from mold Changes in the steel chemistry before and after vacuum application were as follows:

B eore vacuum .After vacuum therebetween a casting cavity 44. An upper portion of the mold 38 is provided with a riser opening 46 between the atmosphere and the casting cavity 44, and a lower portion of the mold is seated on the upper surface of a ange 48 formed on the upper end of the pouring tube 36. A gate 50 in the mold 38 communicates -with casting cavity 44 and is substantially aligned to communicate with the bore 52 of the pouring tube 36. In operation, pressurized pneumatic fluid is forced into tank 10, twhich raises the molten metal 14 upwardly through the pouring tube 36 to fill the casting cavity 44.

It should be noted that normally a thin layer of slag 54 will be present at the top of the molten metal 14, composed of the reaction products of the treatment operation and any unremoved furnace slag. After treatment, a thin slag layer has been found to have a beneficial elect, in that the melt is protected from substantial atmospheric recontamination during subsequent operations.

In summary, the molten steel is tapped into a ladle and subjected to vacuum. An inert gas, such as argon, is passed upwardly from a lower portion of the ladle, in order to stir the melt. The prescribed amount of titanium, columbium, or tantalum is added under vacuum and stirred into the melt during the degassing operation. The molten metal is then immediately pressure poured, which comprises forcing the molten metal generally upward through a pouring tube and into a mold by the use of superatmospheric pressure imposed against the molten steel in the ladle.

In order to further illustrate the present invention, the following examples are given:

EXAM PLE 1 An iron base charge of about 17,000 pounds `was melted and finished in an electric furnace, in the normal manner for producing 300 series stainless steels. After removing the slag in the furnace, the heat was tapped into a ladle and subjected to vacuum for 9.75 minutes. During vacuum treatment, argon gas was bubbled from a porous plug near the bottom of the ladle at a rate of 46 c.f.h., and 0.40 percent titanium was added in the form of ferrotitanium under vacuum. The molten steel was then pressure poured into a slab mold, and the slab was subjected to the various tests mentioned below.

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A full width section was cut at about ten inches from the back of the resultant slab for the purpose of microexamination and chemical analysis. The area of one half of the section was analyzed for titanium, and an inclusion count was performed on the other half. The inclusion count was obtained using a microscope With a 16X objective and ASTM grain size eye piece. The number 8 grain size was employed, giving 144 squares in the grid. The number of micro-inclusions that intersected an individual square was counted to evaluate the relative number of such inclusions in a given area of the slab. The average result, along with corresponding titanium chemical analyses, are illustrated in FIG. 3, which illustrates a Width section of the slab tested from the top downward.

As shown in FIG. 3, the titanium analyses were uniform from the top of the slab downward, with a range of 0.23 to 0.29 percent. 'I'lle number of inclusions found near the top of the slab were substantially uniform near the top of the slab, with no heavily concentrated areas. The total average of inclusions was found to be 6.8.

A photomicrograph, reproduced as FIG. 5, was taken of the area A of FIG. 3 at 100x. As may be seen from this figure, the inclusions were few in number and were evenly dispersed over the entire area.

EXAMPLE 2 In order to compare the results stated in Example 1 with other methods, a similar experiment was performed without the use of vacuum treatment, but with pressure pouring. A charge of steel was melted and nished in a manner similar to that stated in Example l. In this instance, however, the 0.40 percent titanium was added during tap. The melt Was then slagged oif and poured into another ladle in accordance with the conventional reladling technique. Pressure pouring of a slab was then commenced.

FIG. 4 illustrates the inclusion -count and titanium analyses on a Width section of the resultant slab corresponding to that section shown in FIG. 3. As shown in FIG. 4, the number of inclusions was found to be very high near the top of the slab as compared to lower portions, and much higher in total average, which was determined as 2.5.5. The titanium analyses showed a much higher concentration near the top of the slab and an overall range of 0.38 to 1.00 percent.

A photomicrograph, reproduced as FIG. 6, was taken of the area B of FIG. 4, in order to further compare this slab with the one discussed in the previous example. As shown, massive titanium carbonitride agglomerations were found, which would ordinarily render this portion of the slab unsuitable for further use.

It may thus be seen that a novel method for producing certain grades of stainless steel has been discovered, which affords a substantial and unexpected improvement over presently known methods, especially in terms of prevention of excessive and concentrated areas of carbonitride inclusions.

We claim:

1. The method of producing stainless steel having a minimum of carbonitride inclusions containing at least one of the elements selected from a group consisting of titanium, columbium, and tantalum, which comprises melting iron-base materials in the absence of said elements, giving a melt having an undesirably high nitrogen level, removing a substantial portion of the slag from the thus formed iron-base melt, treating said lbath to lower the nitrogen content thereof, then adding at least one of said elements to said melt while agitating the melt in sufficient quantities to promote carbonitride particle formation With said elements and to promote agglomeration of said particles and absorption thereof by remaining slag at the top of the melt, and thereafter pressure pouring said melt by applying superatmospheric pressure thereto.

2. The method according to claim 1 wherein t-he steps of treating and agitating said melt comprises subjecting said melt to a vacuum while bubbling an inert gas upwardly therethrough.

3. The method of producing stainless steel having a minimum of carbonitride inclusions containing a maximum of 0.08% carbon, 2.00% manganese, 1.00% silicon, 0.045% phosphorus, and 0.030% sulphur, and from about 17% to 19% chromium, 9% to 12% nickel, and titanium in an amount of at least ve times the amount of carbon, which comprises providing an essentially slagfree iron-base melt which includes an undesirably high nitrogen content and containing all of said elements except titanium, degassing said melt to lower the nitrogen content thereof, adding said titanium during said degassing and thereafter disposing a tube Within said melt near the bottom thereof, and applying superatmospheric pressure to the top of said melt in order to force said melt upwardly through said tube.

4. The method of producing stainless steel having a minimum of carbonitride inclusions containing at least one of the elements selected from the group consisting of titanium, columbium, and tantalum, which comprises melting iron-base materials in the absence of said elements, removing a substantial portion of slag from the thus formed iron-base melt, degassing said bath to lower the nitrogen content thereof, and then adding at least one of said elements to said melt in sucient amount to promote said carbonitride formation while agitating the melt.

References Cited UNITED STATES PATENTS 2,255,482 19/ 1941 Daeves 75--123X 2,738,267 3/ 1956 Pakkala 7'5--124 2,806,782 9/1957 Faber 75-58X 3,206,302 9/ 1965 Finkl 75-49 L. D'EWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner U.S. Cl. X.R. 

